Tech level and such can be found in the second post.
It resembles the Warring States period in Japan, or the Eberron Last War for a fictitious example. Point is, it's been going on between multiple city-states and so on (big kingdoms have, for the most part, yet to become much of a thing) for a very long time now.
Each PC is a shaman, used to support a community with their magic, and act as a superweapon of sorts in the war. Basically, every nation worships the local nature spirits and minor deities (more or less interchangeable).
A shaman, on the other hand, can actually make use of this, by making a contract with a spirit. Only up to one at a time. The spirit MAY give them some appropriate magic. Usually does, but not much. On their own, spirits just use some helpful but slow magic. This makes many communities in otherwise impossible locations pretty doable.
The shaman also has an "avatar form", where they temporarily act as a host to the spirit, giving the two joint control (sort of, I'll elaborate later) over the resulting form, which is the spirit's body. This can be a lava giant, a thundercloud, a roc, a huge spider made of ivy, a human-sized figure made of light, a swarm of locusts, whatever, depending on the spirit. Generally, the shaman controls this body. The spirit CAN, unless it's weak or the shaman is very powerful, take over at any moment. They just don't because, you know, no point. They're working together.
There is, however, a catch anyway. Human bodies are not meant to house gods. Keep the avatar form up for more than a couple minutes and you'll get incredible fatigue/possible illness, or just outright pass out as well if you keep it up for longer. In fact, it's technically possible to die from this. The spirit will almost certainly leave by themselves before this happens, though. Obvious thing to do. So in practice it never happens.
A shaman still has a fairly shortened lifespan, though. This is why there aren't many, combined with the need for strong mental/physical fortitude, and the ability to bargain effectively with spirits. There's no actual rare ability required. Just not many volunteers and even fewer acceptable ones.
Prolonged transformations can have mental effects. Say you turn into a spider for twenty minutes. Physically, you then go back to being human. Mentally? Mostly, but not entirely. Not for a while, at least.
On the typical belief system: It's pretty much spirit worship wherever you go, at least for anywhere remotely nearby. Actual customs and rituals will vary by the ones in that particular village. Though in practice, it's part worship, part business - the whole shaman thing kind of represents that. You give a fair bit to the spirits, yes, but you definitely expect their help in return.
And spirits change hands with the places they're tied to, which adds a new significance to land grabs/taking over a given town. Assuming the previous shaman is dead or releases them, that is.
Typically, they won't have as much of a say in the new community till they've been there for a while, but they're still quite well-respected. The general consensus is that almost no spirits are bad, as such. Some just end up in the wrong hands, and it's not really something anyone should fault them for. For the average, non-shaman person? Fairly simple. Just take advantage of the benefits, make the odd offering, and that's about it.
Shaman vs. Priest (vs. Capcom): A shaman is a priest. A priest is not necessarily a shaman.
Think of the role as... hm. The difference between, in D&D, a priest (important member of a faith) and a cleric (miracles and blunt trauma everywhere).
Basically, a priest is important, knows all the rites and such, and would probably be quite good at talking to spirits, knowing all the rules, that sort of thing, and will probably be teaching a new shaman about these things. A shaman is chosen based less on knowledge and more on the mental/physical fortitude the job requires. They're the ones that actually fight regularly, use what little magic they're given to help their community, and so forth. In a way, it's a sort of scholar/practical miracle worker division.
The average shaman will be slightly less well-versed in all the proper rules and intricate rituals, but will obviously have a very good idea of all the stuff that's immediately relevant to them. Also, rites being attended by a shaman will be somewhat rare, because they add nothing over the inclusion of a priest and, all else aside, have more immediately practical things to be doing.
Shamans do, however, have one advantage: If you want to know something about spirits, you can frequently just ask the one that's with you and get an immediate answer.